Influence of magnesia in the infiltration of magnesia-spinel refractory bricks by different clinkers

In cement production, which involves the production of cement clinker in rotary kilns, the main refractories used are magnesia-spinel bricks. These bricks may suffer infiltration by the clinker liquid phase, resulting in the corrosion of the spinel and the formation of low refractoriness mineralogical phases, such as the Q phase (C20A13M3S3), which compromises refractory performance. Thus, the aim of this work is to correlate the infiltration resistance of magnesia-spinel bricks made from different grades of magnesia by clinker collected in three different cement plants (A, B and C). The purity of magnesia, besides its physical properties, strongly influences the properties and the infiltration resistance of magnesia-spinel bricks; as such the use of high grade magnesia is essential for producing high performance refractories.


Introduction
The refractory materials include a wide range of oxide or mixture of oxides as well as other materials such as carbon, carbides, nitrides and borides.These materials exhibit superior physicochemical, thermodynamic and structural properties at elevated temperatures, such as a high melting point/refractoriness, resistance to chemical corrosion in an aggressive media, and structural stability (Liu et al., 2013).
The largest customer of the refractory industry is the steel industry, with 70% of the total world production, followed by cement and lime industries, with 7% of refractory production for these markets (Mourão, 2007).In the cement industry, the manufacture of Portland cement involves the steps of grinding the raw material (clay, limestone, bauxite, etc.), homogenization of the raw meal, clinkerization (sintering of the raw meal forming clinker) in rotary kilns, cooling, and grinding of clinker.The clinker produced has a typical composition of 67% CaO, 22% SiO 2 , 5% Al 2 O 3 , 3% Fe 2 O 3 and 3% other components, and four main mineralogical phases identified as C 3 S (3CaO.SiO 2 ), C 2 S (2CaO.SiO 2 ), C 3 A (3CaO.Al 2 O 3 ) and C 4 AF (4CaO.Al 2 O 3 .Fe 2 O 3 ) (Taylor, 1990).
Magnesia-spinel refractory is widely used in upper transition, burning and lower transition zones of rotary kilns in the clinker production and replaced the magnesia-chromite refractory due to environmental issues relating to the formation of Cr 6+ , which is considered toxic (Szczerba et al., 2007).These refractories have two main mineralogical phases: periclase (MgO) and spinel (MgO.Al 2 O 3 or MA).
The spinel is traditionally added between 5 and 30% by weight to magnesiaspinel refractory, which corresponds to an alumina content between 3 and 20wt.%,approximately.According to Ghosh et al. (2004), among the studied concentrations of 10, 20 and 30wt.% of spinel added to the magnesia-spinel refractory matrix, there is a content of 20% optimized properties, such as refractoriness under load, thermal shock resistance and hot modulus of rupture.
Magnesia-spinel refractory exhibits excellent performance in most rotary kilns.However, premature wear can occur due to fluctuations in operating conditions.The main wear mechanisms are infiltration by volatile compounds, infiltration by the clinker liquid phase and mechanical stress.
In the case of infiltration by the clinker liquid phase, CaO from C 3 S peritectic decomposition (C 3 S → C 2 S + CaO at 1250 °C) reacts with Al 2 O 3 of the spinel forming mayenite phase (C 12 A 7 ) in the temperature range between 1000°C and 1350°C, with the probable mechanism indicated by Equation 1 (Gonçalves and Bittencourt, 2003;Wajdowicz et al., 2011): 7 MA (brick) + 12 C 3 S (clinker) + liquid phase → C 12 A 7 + 7 M + 12 C 2 S + liquid phase The mayenite is an intermediate phase, which, in the absence of SO 3 , leads to the formation of the Q phase (C 20 A 13 M 3 Si 3 or Ca 20 Al 26 Mg 3 Si3O 68 ) between 1300 °C and 1400°C, with probable mechanism indicated by Equation 2 (Gonçalves and Bittencourt, 2003;Wajdowicz et al., 2011): Mayenite and the Q phase are low refractoriness phases, which compromise the performance of the refractory in the rotary kilns.Rodríguez et al. (2012) reported the excellent resistance to the clinker of bricks based on sintered magnesia and electrofused magnesiacalcium zirconate (MgO-CaZrO 3 ) using spinels of magnesium aluminate (MgAl 2 O 4 ) and hercynite (FeAl 2 O 4 ) in the refractory matrix.
According to our knowledge, the relation between refractory raw materials and infiltration resistance is not found in literature.Liu et al. (2014) investigated the composition and microstructure of a periclase-composite spinel brick used in the burning zone of a cement rotary kiln and compared to the original brick.The results indicate that cement clinker and alkali salts are two important agents that cause corrosion especially of the bonding phase of refractory in cement rotary kilns.
The objective of this study is to correlate the infiltration resistance of magnesia-spinel refractory bricks made from different grades of magnesia by the clinker liquid phase, which is a gap in the literature about refractory bricks of magnesia-spinel.

Materials and methods
Two types of sintered magnesia (type 1 and 2) and an electrofused spinel were used.Raw materials were characterized regarding bulk density (BD) and apparent porosity (AP) according to the ABNT NBR 8592 standard.The chemical analysis was performed by X-ray fluorescence using a PW2540 Philips spectrometer, the X-ray diffraction analysis was performed using a PANalytical, model X'Pert PRO device, and the analysis was performed in the X'Pert HighSore Plus program using the JCPDS -International Centre for Diffraction Data as database.The Zeiss AXIO imager reflected light optical microscope was used to evaluate the microstructure of the sintered magnesia.
Table 1 shows the compositions of magnesia-spinel bricks produced in the laboratory.Thirty kilograms of each composition was mixed for 15 minutes on a roller mixer with the aid of an organic binder.Bricks of 160 mm x 85 mm x 64 mm in dimensions were pressed on a laboratory hydraulic press with pressure of approximately 150 MPa, which had passed through pre-drying at 120 °C for 12 hours, and oxidant firing at 1500 °C for 5 hours in a Bickley gas oven.
After heat treatment, the bricks were characterized in relation to bulk density (BD) and apparent porosity (AP) according to the ABNT NBR 6220 standard; elasticity modulus at room temperature (EM) according to the ASTM C885 standard; cold crushing strength (CCS) according to the ABNT NBR 6224 standard; hot modulus of rupture (HMOR) at 1200 °C for 3 hours according to the ASTM C583 standard; abrasion according to the ASTM C704 and permeability according to the ASTM C577 standard.The infiltration test by clinker liquid phase was performed adopting a procedure similar to that of Kozuka (1993) testing, and was performed in a laboratory rotary kiln, as shown in Figure 1.The Kozuka testing was conducted at 1800 °C where 400 grams of clinker were added 5 times to the kiln, at an interval of 30 minutes, for a total addition of 2000 grams.After the test, the samples of 100 x 60 mm x 90 mm x 50 mm in a trapezoidal shape, were cut into 6 slices for chemical analysis, starting from the hot face (slice 1) to the cold face (slice 6).

Results
Table 2 shows the properties of the raw materials used.The bulk density (BD), apparent porosity (PA) and chemical purity are different for the two types of magnesia.The magnesia type 1 showed lower BD and higher AP than type 2 and a typical microstructure shown in Figure 2, with a high content of elongated pores.Type 2 magnesia showed higher BD and lower AP than type 1 and a typical mi-crostructure shown in Figure 3, with a reduced amount of pores.The bulk density of spinel is higher than the bulk density of magnesia due to the electrofusion process involving temperatures around 2000 °C.

Raw material
Magnesia    Table 3 lists the properties of the compositions A-1 and A-2 after heat treatment at 1500 °C for 5 hours.All found properties were consistent with literature (Sczzerba et al., 2007;Rodríguez et al., 2013) and also with industrial production data.The composition A-2, produced with type 2 magnesia, exhibited superior properties than the composition A-1, with higher BD, CCS and HMOR and lower AP, abrasion and permeability.The clinkers collected from three different cement factories (A, B and C) are characterized and results are shown in Table 4.The clinkers presented all of the clinker phases and similar contents of CaO, SiO 2 , Al 2 O 3 and Fe 2 O 3 , but the clinker collected in factory B showed the highest content of impurities such as K 2 O and SO 3 .

Discussion
The chemical analysis of magnesia presented in Table 2 shows that the type 1 magnesia had a higher level of impuri- The Type 2 magnesia shows the value of 3.2 for the CaO/SiO 2 molar ratio with the presence of minority phase larnite (βC2S) of high refractoriness.The spinel composition is not stoichiometric (28.2 wt% MgO and 71.8 wt% Al 2 O 3 , Szczerba et al., 2007), showing unreacted MgO.Excess of MgO is one way to ensure that there is no formation of in situ spinel during firing of magnesiaspinel bricks, since it is an expansive reaction (approximately 8% in volume), which may damage the mechanical strength of the refractory (Nakagama et al., 1995).
Considering the transition zones of the cement rotary kiln, the superior characteristics of composition A-2, shown in Table 3, contribute to the better performance of the brick made with Type 2 magnesia.These results are in agreement with those obtained by Szczerba et al. (2007), who studied the influence of the physicochemical properties of magnesia on the final properties of magnesia-spinel products containing 8 or 18 wt% of electrofused spinel.Szczerba et al. (2007) reported that compositions containing sintered magnesia of high purity and superior physical properties achieved more suitable properties.
The results of the infiltration test of compositions A-1 and A-2 with clinkers A, B and C illustrate a classical phenomenon known as silicate migration.In steelmak-ing and non-ferrous industrial processes, due to the thermal gradient and operating conditions, the infiltration of slag rich in silicates (C 3 S and C 2 S) occurs in the open pores of the refractory, and this infiltration is more intense if the refractory contains a higher content of impurities, with formation of low refractoriness phases (Havranek, 1967).
The physicochemical properties of magnesia have a great influence on the properties and infiltration resistance of magnesia-spinel refractory bricks by the clinker liquid phase.Therefore, the use of high grade magnesia with a high purity, high bulk density and low apparent porosity leads to the production of magnesiaspinel bricks of high performance.

Conclusions
This investigation evaluated the influence of the physicochemical properties of magnesia on the properties and infiltration resistance of magnesia-spinel refractory bricks by the clinker liquid phase.The use of magnesia with low impurity content, presence of minority phase of high refractoriness, high bulk density and low apparent porosity improved properties and infiltration resistance.Therefore the use of high grade magnesia is essential for the production of high performance refractory.

Figure 1
Figure 1 Rotary kiln used in the infiltration test.For the infiltration test, clinkers of three different cement factories (A, B
The results of the infiltration test by clinker liquid phase are shown in Figures 4 to 6 indicating the infiltration of CaO and SiO 2 , which are the most relevant oxides, along the hot face (slice 1) to the cold face (slice 6) of the samples from A-1 and A-2 compositions.The composition A-1, produced with type 1 magnesia, showed the highest level of CaO and SiO 2 infiltration, independently of the clinker used in the test (A, B or C).

Figure 4
Figure 4 Infiltration of CaO e SiO 2 from the hot face (slice 1) to the cold face (slice 6) after infiltration test by clinker A.

Figure 5
Figure 5 Infiltration of CaO e SiO 2 from the hot face (slice 1) to the cold face (slice 6) after infiltration test by clinker B.

Figure 6
Figure 6 Infiltration of CaO and SiO 2 from the hot face (slice 1) to the cold face (slice 6) after infiltration test by clinker C.
ties (SiO 2 , Fe2O 3 and MnO), with a lower MgO content in relation to the Type 2 magnesia.Due to the high content of SiO 2 of the Type 1 magnesia, the CaO/SiO 2 molar ratio has a value of 0.4 which determines the presence of minority phases

Table 2
Properties of the raw materials.

Table 4
Loss on ignition, chemical composition (wt.%) and phases identified by XRD of clinkers.